The trend is that in future battery systems will be used increasingly both in stationary applications and in vehicles such as hybrid vehicles and electric vehicles. In order to be able to meet the demands which are made for a respective application in terms of voltage and available power, a large number of battery cells are connected in series. Since the current provided by such a battery must flow through all the battery cells, and a battery cell can conduct only a limited current, battery cells are often additionally connected in parallel in order to increase the maximum current. This can be done either by providing a plurality of cell windings within a battery cell housing or by externally interconnecting battery cells. However, one problem in this case is that compensation currents between the battery cells which are connected in parallel may occur on account of cell capacitances and voltages which are not exactly identical.
FIG. 1 illustrates the basic circuit diagram of a conventional electric drive unit as is used, for example, in electric and hybrid vehicles or else in stationary applications such as for rotor blade adjustment in wind power plants. A battery 10 is connected to a DC voltage intermediate circuit which is buffered by a capacitor 11. A pulse-controlled inverter 12 is connected to the DC voltage intermediate circuit and provides sinusoidal voltages, which are out of phase with respect to one another, at three outputs via in each case two switchable semiconductor valves and two diodes for the operation of an electric drive motor 13. The capacitance of the capacitor 11 must be large enough to stabilize the voltage in the DC voltage intermediate circuit for a period of time in which one of the switchable semiconductor valves is connected. In a practical application, such as an electric vehicle, a high capacitance is obtained in the mF range. Owing to the usually very high voltage of the DC voltage intermediate circuit, such a high capacitance can be realized only at great expense and with a large requirement in terms of space.
FIG. 2 shows the battery 10 from FIG. 1 in a more detailed block diagram. A multiplicity of battery cells are connected in series and optionally additionally in parallel in order to achieve a high output voltage and battery capacity which are desired for a respective application. A charging and disconnection device 16 is connected between the positive pole of the battery cells and a positive battery terminal 14. Optionally, a disconnection device 17 can additionally be connected between the negative pole of the battery cells and a negative battery terminal 15. The disconnection and charging device 16 and the disconnection device 17 each comprise a contactor 18 and, respectively, 19 which are provided for disconnecting the battery cells from the battery terminals 14, 15 in order to de-energize the battery terminals. Otherwise, there is considerable potential danger to servicing personnel or the like on account of the high DC voltage from the series-connected battery cells. A charging contactor 20 with a charging resistor 21 connected in series with the charging contactor 20 is additionally provided in the charging and disconnection device 16. The charging resistor 21 limits a charging current for the capacitor 11 when the battery is connected to the DC voltage intermediate circuit. For this purpose, the contactor 18 is initially left open and only the charging contactor 20 is closed. Once the voltage at the positive battery terminal 14 reaches the voltage of the battery cells, the contactor 18 can be closed and the charging contactor 20 may be opened. The contactors 18, 19 and the charging contactor 20 increase the costs of a battery 10 to a considerable extent since stringent demands are made of them in respect of reliability and the currents to be carried by them.
The series connection of a high number of battery cells means that, in addition to the high overall voltage, there is the problem that the entire battery fails if a single battery cell fails because the battery current has to be able to flow in all the battery cells owing to the series connection. Such a failure of the battery can lead to a failure of the entire system. In the case of an electric vehicle, a failure of the drive battery causes the vehicle to become immobile, and in other devices, such as for example the rotor blade adjustment in the case of wind power plants this can even lead to undesired situations in the case of strong wind. For this reason, a high level of reliability of the battery is advantageous. According to the definition, the term “reliability” means the ability of a system to operate correctly for a predefined time.